The present invention relates to electric power tools and in particular to a method and apparatus for controlling the rotational speed of portable electric power tools, for example an electronically-controlled drill, hammer-drill, rotary hammer, and the like, in both the forward and reverse directions.
It is known to provide a speed control circuit for a portable electric power tool which provides the user with the ability to control the forward speed of rotation in a continuous manner from a preset minimum, which may be zero, to a predetermined maximum.
Conventional portable electric power tools typically employ universal motors whose commutator brushes are shifted slightly from the neutral position to provide better commutation in the forward direction of rotation and better torque characteristics. However, with such motors, if a reversing switch is provided that simply reverses the connections to the brushes, poor commutation and poor torque characteristics will result when operated in the reverse direction.
Accordingly, it is the primary object of the present invention to provide a speed control circuit for an electric power tool which is adapted to control the speed of rotation of the electric motor in both the forward and reverse directions and in which the speed control circuit is adapted to limit the rotational speed of the motor in the reverse direction to keep commutation problems to an acceptable level.
Furthermore, it is an object of the present invention to provide a reversing speed control circuit for an electric power tool that is relatively simple, economical, and convenient to install during assembly of the tool.
According to the present invention, a power tool driven by an electric motor includes a speed control circuit for enabling a user to selectably control the forward speed of rotation of the motor, means for reversing the direction of rotation of the motor, and a speed control and limiting arrangement operable when the direction of rotation of the motor is reversed to automatically limit the speed of rotation of the motor to a predetermined maximum speed while allowing variation up to the maximum speed.
In the preferred embodiment, a common control means is provided for controlling the rotational speed of the motor in both the forward and reverse directions. In particular, a set speed signal is developed in accordance with the desired speed selected by the user which is compared to an actual speed signal generated by a tachogenerator coupled to the rotor shaft of the motor. The control means is adapted to control the firing angle of a triac connected in series with the motor across the main a.c. supply line to control the speed of the motor in accordance with the result of the comparison. The speed limiting arrangement is operative when the motor is operated in the reverse direction to limit the magnitude of the set speed signal to a predetermined maximum value to thereby prevent the rotational speed of the motor from exceeding a preset level.
Additionally, the speed control circuit may also include means for sensing a change in the condition of the reversing switch and removing power from the motor until its speed drops to a relatively low level, or completely to zero, before re-applying power in the reverse direction.
The speed control circuit according to the present invention may operate on an analog or a digital basis and may incorporate a microprocessor programmed to effect the control functions described in detail below.
The speed control circuitry is preferably contained within a compartment in the housing of the power tool through which air is circulated by the cooling fan coupled to the output shaft of the motor to extract heat generated by the components of the circuit. The compartment may be isolated from the motor by a barrier which may also reduce heat transmission from the motor to the compartment.
The tachogenerator utilized in the preferred embodiment to monitor the rotational speed of the motor comprises a permanent magnet, which may be a ring magnet, secured to the rotor shaft of the motor for rotation therewith and a Hall effect device mounted on a carrier fixed with respect to the casing of the power tool. The poles of the permanent magnet are located at its periphery. The carrier has a peripheral flange on which the Hall effect device or pick-up coil is mounted and which encircles the permanent magnet being separated therefrom by a radially extending gap.
Advantageously, the magnet and pick-up device are located at the commutator end of the rotor shaft at a position outboard of the bearing supporting the end of the shaft to facilitate mounting of the magnet and pick-up device during assembly of the tool.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: